I think it's important to write IC characters, but it's not necessarily the most important thing. It's great if you can mimic the writing style of the author, but I think just basic quality and good writing is more important - at the end of the day, it's all just about messing around in the worlds you love, and there's lots of funny crack and AU fics out there that don't necessarily have IC characters - take Harry Potter and the Methods of Rationality for one! It's one of the most popular Harry Potter fanfics ever written, and there are barely any characters who act IC.